The present invention relates to a radiator support apparatus for a vehicle, which causes a radiator to serve as a dynamic damper so as to eliminate a vehicle body vibration in a low-engine speed state.
Conventionally, as disclosed in, e.g., Japanese Patent Laid-Open No. 57-84223, in order to eliminate a so-called idling vibration or a vehicle body low-frequency vibration such as low-frequency noise caused by a variation in rotation in an engine idling state, a dynamic damper is constituted by utilizing the mass of a radiator mounted in the front end portion of a vehicle body, thereby eliminating a vertical flexural vibration in the back-and-forth direction of the vehicle body. A dynamic damper of this type is normally constituted in such a manner that the lower end portion of a radiator is attached to a vehicle body using two, i.e., right and left mount members comprising elastic members, and the upper end portion of the radiator is also attached to the vehicle body using two mount members. In this case, the upper and lower mount members are set to have a spring constant, so that the dynamic damper can provide its effect in a low-engine speed state.
The flexural vibration in the back-and-forth direction of the vehicle body normally has a vertical vibration mode which exhibits a maximum amplitude at the front end of the vehicle body. Therefore, when the dynamic damper is constituted as described above, the radiator defining the damper mass is vertically vibrated at the maximum amplitude position of the flexural vibration. For this reason, when damper characteristics are properly selected, the flexural vibration of the vehicle body in the low-engine speed state can be canceled by bouncing.
In this manner, when the vehicle body low-frequency vibration is eliminated by the dynamic damper utilizing the mass of the radiator, the vertical flexural vibration in the back-and-forth direction of the vehicle body is conventionally assumed as the vehicle body low-frequency vibration to be eliminated. However, in a front wheel drive vehicle, in which heavy members are concentrated on the front, end portion of the vehicle body, it has recently been found that a so-called torsional vibration caused by a vertical vibration of the front end portion of the vehicle body in the widthwise direction of the vehicle body occurs in addition to the flexural vibration in the back-and-forth direction of the vehicle body.
FIG. 1 shows vehicle body low-frequency vibration characteristics in a front wheel drive vehicle. In this case, as can be seen from FIG. 1, vibration peaks are present at a frequency of about 22 Hz (corresponding to an engine speed of 660 rpm) of the vehicle body vibration, and at about 27 Hz (corresponding to an engine speed of 750 rpm). The peak at 22 Hz corresponds to that of the torsional vibration, and the peak at 27 Hz corresponds to that of the flexural vibration. However, in the conventional dynamic damper, the right and left mount members at the lower end portion of the radiator have the same spring constant. For this reason, the dynamic damper characteristics are as represented by a solid curve in FIG. 8. More specifically, a pitching resonance 1 is close to a bouncing resonance 2. Therefore, when the bouncing resonance is set at about 27 Hz so as to eliminate the flexural vibration, the pitching resonance appears at about 25 Hz, and the torsional vibration at about 22 Hz cannot be eliminated.